1. Field of the Invention
The subject matter of this patent relates generally to the art of drilling large diameter shafts in the earth and, more particularly, to the expeditious removal of cuttings from the shaft bottom during the drilling operations.
2. Description of the Prior Art
Large diameter shafts are used to provide general access to an ore body in mineral mining practice.
Large diameter shafts are commonly drilled with rotary drill bits which comprise a main bit body having a plurality of cutter assemblies rotatively mounted thereto. The cutters contact and disintegrate the earth formation at the bottom of that shaft, thereby producing cuttings or detritus at that location.
Drilling fluids or "mud" are employed to hydraulically purge the drilling surface of cuttings.
Various circulation systems for drilling fluids have evolved.
The "reverse" circulating system is commonly used in drilling large diameter shafts. Mud is supplied to the annulus of the shaft by pumping mud down the outside of the drillstring between the conduit and the shaft wall. The fluid flows across the bit face from the peripheral edge of the bit to the center of the bit to be pumped up the drill column along with the purged cuttings.
Another version of the reverse circulation systems utilizes a dual-string drill column, which is a concentric pipe within a pipe. The mud is pumped down the annular space between the inner and outer conduits, distributed via plumbing within the bit, and then pumped along with the entrained cuttings up the interior of the smaller inside pipe of the drill column.
U.S. Pat. No. 3,471,177 teaches the construction of a dual-string reverse circulation drilling column, however, the patent does not teach its application to a reverse circulation system for a large diameter drill bit.
Present day circulation systems are inefficient for transporting the drilled cuttings across the bottom of the shaft and out of the path of advancing cutters. Consequently the cuttings generally are recut to smaller sizes, until the drilling fluid circulation finally does sweep them away. This repeated recutting is wasteful of drilling energy and is deleterious to cutter life and to rate of penetration of the rock bit.
U.S. Pat. Nos. 4,105,083; 4,195,700 and 4,200,160 describe sweeps or brushes attached to large diameter bits which are intended to meet the problem of moving chips out of the path of advancing cutters.
An advantage of this invention over the prior art is the concentration of drilling mud along an arcuate flow path or channel formed by the bit body to direct drilling mud from a peripheral edge of the bit, across the cutters strategically positioned within the channel and through the center of the bit, thus efficiently moving detritus material from the boreface.
Another advantage over the prior art is the ability to cool and clean each cutter strategically positioned within the arcuate channels formed by the large diameter bit body to prevent "balling" of the cutters, thereby further enhancing the penetration rate of the bit.